Toxic metals may enter a combustion system in many physical and chemical forms, for example, as constituents of a hazardous or municipal solid waste to be incinerated or as trace quantities in coal. In fact, studies have shown that coal combustion and waste incineration are the two major contributors to atmospheric loading of toxic metals such as antimony, arsenic, cadmium, selenium, vanadium, lead and zinc. Once introduced into a combustion environment, a metal may undergo transformations to different phases as well as to different chemical species depending upon combustion conditions and the presence of chlorine and other reactive species. Also, at combustion temperatures, metals may be vaporized and then undergo nucleation to form a submicron aerosol, or the metal vapor may condense onto existing particles. These resulting particles, formed by nucleation or condensation, have been observed to have a diameter of approximately 0.02 .mu.m. Through growth by condensation of vapor or by coagulation with other particles, these particles ultimately may have a diameter of from about 0.02 .mu.m to about 1.0 .mu.m in the flu gas. For example, in one study on hospital waste incineration, a bimodal distribution in the flu gas was observed, and the particles having a diameter between about 0.1 .mu.m and 0.2 .mu.m accounted for 7% to 74% of the lead, 62%-77% of the cadmium, and 20%-80% of the zinc in the total particulate phase. (Kauppinen, E. I. and Pakkanen, T. A., "Mass and Trace Element Size Distributions of Aerosols Emitted by a Hospital Refuse Incinerator", Atmos. Environ., 24A, 423 (1990).
Unfortunately, flu gas cleaning equipment used in combustion systems is least efficient in capturing particles having diameters in the submicrometer size range. For example, electrostatic precipitators are used in many coal-fired combusters and typically exhibit the lowest collection efficiency for particles less than 1 .mu.m in diameter. Particles in these size ranges potentially pose a greater health threat than larger particles since they penetrate deeper into the lungs where the toxic materials come into contact with the blood. This potential adverse health impact of metal emissions from combustion devices is an appropriate incentive to investigate new methods and technologies for metal removal from waste gas streams. Furthermore, the United States Environmental Protection Agency (US EPA) has begun to regulate toxic metal emissions from combusters pursuant to Title III of the 1990 Clean Air Act Amendments which specifically lists eleven metals and their compounds as air toxics.
In an attempt to control such toxic metal emissions, researchers have proposed several control methods using various bulk solid sorbents to chemically adsorb various metals thereby reducing their discharge in particulate form into the atmosphere.
One such method includes combusting a metal contaminated waste in a fluidized bed of sorbent. (Oho, T., Chen, J., Hopper, J. and Oberacker, D., "Metal Capture During Fluidized Bed Incineration of Wastes Contaminated with Lead Chloride", Combust. Sci. and Technol., 85, 101 (1992)). Other proposed methods include injecting a sorbent into the high temperature region of a combustion device (Scotto, M., Petersen, T. and Wendt, J., "Hazardous Waste Incineration: The In-Situ Capture of Lead by Sorbents in a Laboratory Down-Flow Combuster", 24th International Symposium on Combustion, the University of Sydney, Sydney, Australia (1992), and passing a metal vapor at high temperatures through a packed bed of sorbent (Uberoi, M. and Schadman, F., "High Temperature Removal of Cadmium Compounds Using Solid Sorbents", Environ. Sci. Technol., 25, 7, 1285 (1991); and Uberoi, M. and Schadman, F., "Sorbents for Removal of Lead Compounds from Hot Flu Gases", AlChE Journal, 36, 2, 307 (1990)).
Although these bulk solid sorbent control methods may reduce metal emissions, they have several limitations. For example, because of mass transfer limitations with the bulk solid sorbent, the sorbent is not very efficient, and therefore, a large amount of sorbent must be used. More specifically, the metals may react with the surface of the bulk solid sorbent to form a metal-sorbent complex at the surface which plugs the pores of the sorbent and blocks access by the metals to the inner core volume of the bulk solid sorbent. This inhibits mass transfer of metal vapor to a significant active portion of the overall sorbent reactive surface, thereby severely limiting the efficiency of the bulk solid sorbent. In order to overcome this mass transfer limitation, the ratio of sorbent material to metals must be significantly increased thereby increasing not only raw material costs, but also post treatment costs incurred in land filling or otherwise disposing of the resulting large quantities of filtered or precipitated powdered material.
Therefore, it would be extremely desirable to have a method of reducing toxic metal emissions from a combustion system in which the system uses a sorbent material more efficiently, thereby providing enhanced capture and subsequent storage of toxic metals at a reduced cost.